A narrow channel knife-edge door design has numerous disadvantages. Because the knife and channel are made of brass, corrosion occurs and creates non conductive zinc and copper oxides.
In addition, the channel traps water dirt and contaminants, whereby performance degrades exponentially. Also, the channel is extremely difficult to clean. Typically, cleaning requires removal of the finger stock seal in the narrow channel (that is, brass receiving “fingers” that help create an electrical seal with the knife edge in the channel). The finger stock seal, when removed, often gets damaged and cannot be reused. Also, all corrosion has to be removed from narrow channel and knife edge surfaces, which is difficult. A conductive lubricant can be used on the brass surfaces to slow corrosion. However, the silicone lubricant traps and holds dirt and dust particles reducing shielding effectiveness.
Additionally, water freezes in narrow channel rendering the door inoperable in cold climates. Moreover, the knife edge can wear below serviceable limits in dry sandy environments requiring replacement of entire door within 5 years.
Large lever and cam mechanisms are required to open and close the knife edge door. Appreciable wear on the finger stock seal and knife edge occurs because of this mechanical opening action. There are two conditions that make the door difficult to operate: 1—the beryllium copper finger stock seal are heat treated, or tempered, to make them springy. This process also hardens them. When the surface of the finger stock seal begins to wear and become microscopically abraded, it digs in and grabs the softer brass knife edge requiring more and more effort to operate the lever mechanism. This can be visually confirmed by the grooves that each of the fingers eventually wears into the brass knife edge; 2—the lever mechanism only unseats the knife edge on the strike side of the door requiring the operator to manually pull the door's knife edge completely out of the narrow channel and finger stock seal at the top, bottom and hinge side and to push the door in until the lever mechanism can be engaged.
Very high maintenance is required for the narrow channel knife edge door design. In particular, weekly flushing of the narrow channel with solvents is required to remove loose dirt. In addition, weekly lubrication with conductive lubricant of the finger stock seal in the narrow channel is recommended. For the reasons discussed above, monthly or quarterly replacement of finger stock seal occurs—with associated down time—based upon usage of the knife edge door. Finger stock seal replacement requires special tools and takes approximately 1 hour. Moreover, monthly or quarterly lubrication of the mechanical operating mechanism is required based upon usage.
Additional problems with the narrow channel knife edge design arise because the brass knife edge can be bent causing misalignment—which makes the door difficult if not impossible to operate and causes a loss of shielding effectiveness. Similarly, the knife edge at the sill cannot be stepped on as damage will occur. The sill must be protected by a steel plate or wood ramp of sufficient strength if furniture, fixtures or supplies need to be wheeled or carted through the door.